This invention relates to a suspension system for a vehicle and more particularly to a suspension system which comprises a shock absorber of damping force characteristic variable type provided between a sprung member and an unsprung member.
In general, the vehicle suspension system comprises a shock absorber, for damping the oscillation of a wheel, interposed between a sprung member and an unsprung member. There are various types of shock absorbers and shock absorbers of damping force characteristic variable type are grouped into one which damping force characteristic (characteristic of different damping coefficient) is changed into two levels (higher level and lower level) and the another which the damping force characteristic is changed into many levels or changed steplessly.
The basic function of the shock absorber of damping force characteristic variable type is that the damping force characteristic of a shock absorber is set lower (i.e., SOFT side) when the damping force generated by a shock absorber acts in the oscillation-stimulating direction (in the same vertical direction as a sprung member) and the damping force is set higher (i.e., HARD side) when the damping force acts in the oscillation-restraining direction (in the opposite vertical direction to the sprung member) so that oscillation-restraining energy is set larger than oscillation-stimulating energy. Thus, comfortableness to ride in and running stability is improved.
Various methods for judging the direction that the damping force of a shock absorber is acting, either in the oscillation-stimulating direction or oscillation-restraining direction with respect to the sprung member, are proposed. For example, the Japanese Patent Application Laying Open Gazette No. 60-248419 discloses a method for judging a direction of a damping force in such a way: the sign of relative displacement between the sprung member and the unsprung member is checked whether it is same as a sign of differentiated value of it, which is relative speed between the sprung member and the unsprung member. If signs are same, it is observed that damping force is acting in the oscillation-stimulating direction and if signs are not same, it is observed that damping force is acting in the oscillation-restraining direction. Also, the Japanese Patent Application Laying Open Gazette No. 61-163011 discloses a method, in which a sign of absolute speed of the sprung member is checked whether it is same as a sign of relative speed between the sprung member and the unsprung member. If signs are same, it is observed that damping force is acting in the oscillation-restraining direction and if signs are not same, it is observed that damping force is acting in the oscillation-stimulating direction.
However, in a vehicle having a shock absorber of damping force characteristic variable in multisteps type, when the damping force characteristic is controlled to generate approximately ideal damping force, a loud noise or oscillation is caused by the large and quick changes of the damping force characteristic of the shock absorber.